fix(TreeData): addItem should keep current sorted column

[ghiscoding]

• prior to this PR, calling addItem(), or addItems(), was always resorting using the Tree Data initial sort because the method being called is the same for the first build of the Tree Data and/or adding an item. So when we're adding an item, we should really reuse any existing sort that may exist when resynching the flat/hierarchical and resorting them (we always need to resort whenever it changes because by SlickGrid doesn't support Tree Data, we only support it by keeping copies of both flat & hierarchical and then resort them whenever something changes)

[stackblitz[bot]]

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